Background for the subject technology can be found in the pioneering work by the late Carl Wagner [32] on the active-passive oxidation of Si and the work by Lou, Mitchell and Heuer [33] on treatment of volatility diagrams.
The replacement of aluminum with copper as an interconnect material in ULSI structures is being actively considered in the semiconductor industry [1–7]. Compared to aluminum, copper has a lower electrical resistivity and significantly larger electromigration resistance. The utilization of copper interconnects is expected to improve chip performance due to lower RC time delays and power dissipation. Two basic processes for copper-based interconnect structures have been proposed [2–6]. The first is the dual damascene method, which has been recently introduced in IC manufacturing by IBM and others [2]. In this method for producing multilevel structures, the dielectric layer is first deposited on the silicon substrate. The via pattern is then formed by dry etching of the dielectric. A thin diffusion barrier (e.g., TaN) is subsequently deposited on the patterned dielectric followed by deposition of copper using blanket CVD, PVD or electroplating techniques. The excess copper on top of the features is finally removed by a process of chemical mechanical polishing (CMP).
The second process for the patterning of copper is the dry etch method [3–5], which is based on the process used for the patterning of aluminum [8–10]. In this process, copper is first deposited on the dielectric having a barrier layer using, for example, a sputtering process. A hard mask or photoresist is applied over the copper and the pattern is etched using a dry etch process such as reactive ion etching(RIE). The structure is then filled with dielectric and planarized using CMP.
Unfortunately, unlike aluminum, the anisotropic dry etching process for copper using halides is difficult at low temperatures due to the lower volatility of copper based compounds [5, 11–14]. At wafer temperatures of 200° C. or higher, reactive ion etching of copper in the presence of chlorine can be achieved, but the anisotropy is lost due to problems associated with the use of organic mask layers. Inorganic mask layers have been utilized, however they appear to provide less etch anisotropy due to the lack of protective sidewall films. Higher temperatures have a negative effect on the electrical conductivity of copper due to the increased solubility of chlorine in copper and can also cause significant problems due to dopant diffusion and stress on equipment and components during manufacturing. Redeposition of the copper chlorides from the warm exhaust gas stream on cooler equipment components is a serious issue associated with high temperature etching. The continued corrosion of the patterned copper due to the precipitation of copper chloride compounds (15) on cooling to room temperature is yet another problem. Thus lower etching temperatures are desirable.
The continued corrosion of the patterned copper during the etch process itself has been reported to be an important problem. One would expect that this problem can be easily resolved by ensuring that the nonvolatile copper chlorides are removed on completion of the etch to prevent further corrosion. However this problem needs to be considered carefully. The existing dry etch processes for Cu are usually carried out at higher temperatures (typically>200° C.). Hence the solubility of chlorine in copper is much higher or alternatively the diffusion depth of chlorine in copper is significant compared to lower temperatures. On cooling down to room temperature, the solubility of chlorine in copper is reduced and as a result the excess chlorine may precipitate in the form of copper chlorides causing further corrosion. To prevent this from occurring, the residual chlorine in the etched copper has to be removed by exposing to a reducing atmosphere such as hydrogen or by further etching of the copper containing the dissolved chlorine without the use of chlorine, in effect using natural volatilization (a process which is extremely slow). Such a process besides causing problems with dimensional control would require a significant amount of time, since it is known that the diffusivity of chlorine in copper is very low (perhaps more than two orders of magnitude lower than that of copper at room temperature [15]). A dry etch process for copper at lower temperatures besides being beneficial in relation to the thermal cost, has the advantage that the diffusion depth of chlorine in copper is reduced, and hence corrosion problems due to the reasons stated earlier are expected to be less severe. Hence, the development of a cost-effective, low-temperature, dry etch process for copper is highly desirable. This appears to be elusive at the present time.
An important advantage of the damascene process is that the copper does not have to be patterned directly and so the problems associated with the dry etching of copper are avoided. The number of processing steps, as compared to the dry etching process, is also reduced [2]. However, there are some challenging issues associated with the Cu deposition for very small feature sizes (less than 0.25 μm) and high aspect ratios [16,17] as well as difficulties associated with the copper CMP process [16,18–19] and the safe disposal of the byproducts of the electroplating process [20,21]. The practical problems and costs associated with the transition to the new technology based on the copper damascene process are still being evaluated with copper CMP regarded as the major bottleneck [16,19,22]. Furthermore, the introduction of porous, low dielectric constant materials is expected to pose significant challenges to the copper CMP process in the near future [18].
In comparison, if a cost-effective, low temperature dry etching process for copper can be developed, many of the problems associated with copper damascene can be avoided. Besides, the transition may be relatively easier and cheaper since essentially the same equipment and processes used for the patterning of aluminum can be utilized. An obvious benefit of the dry process, as compared to the damascene process, is that there is considerable experience in handling of the disposed gases using safe and environment-friendly procedures. It has been recently reported [12] that dry etch patterning can be readily combined with air gaps, which is an important advantage over the damascene process, since air gaps reduce capacitance and leakage current between lines. An added benefit of this innovation is that no barrier films on the sidewalls of the copper lines are needed.
In spite of the difficulties with copper reactive ion etching, several approaches have been proposed. Most of these utilize temperatures over 150° C. to improve the volatility of the copper chloride gas species. Winters [23] suggested a two-step process, that first required the formation of CuCl at room temperature followed by heating between 150–200° C. to desorb Cu3Cl3(g), which he identified as the primary desorbing species. Arita et al. [5] and Igarashi et al. [24] used a gas mixture of SiCl4, Cl2, N2, and NH3 for dry etching and used a resist mask by hard-baking above 250° C. Since copper was oxidized during the ashing process for resist removal, they suggested a CVD SiO2 film or a plasma CVD nitride mask which could remain in place after the RIE. A thin SiON film that formed on the sidewall acted as a suitable barrier during the etching process. Miyazaki et al. [14] were able to obtain an anisotropic etch profile using chlorine as the only reactant. The operating temperature (230–270° C.) and the partial pressure of chlorine (0.3–1.3 Pa) had to carefully controlled in their process. Markert et al. [12] used an ICP system containing a chlorine-based gas mixture (10–20 mtorr) along with Ar and N2 at a wafer temperature of 250° C. CH4 was used to protect the sidewall during the etch. Ye et al. [25] utilized a mixture of HCl or HBr along with hydrogen as the reactant gases at temperatures greater than 150° C. Jain et al. [26] used a two-step process for the isotropic etching of copper, first by oxidizing copper using hydrogen peroxide followed by its removal using hexafluoroacetylacetone (hfacH). A continuous process using the same reactants at 150° C. was also proposed by them. Lee et al. [27] performed a RIE etch using a CCl4/N2 electron cyclotron plasma (ECR) at temperatures above 210° C.
Low temperature methods for the etching of copper usually require some form of radiation such as ultraviolet [28–29], infrared [30] or laser [31], to enhance the volatilization of the etch products. It is reported that these do not have good etch uniformity for large-area substrates and the processes are not easy to control and maintain [32]. Kuo and Lee [32, 33], and Allen and Grant [34] suggested a process in which CuClx was intentionally formed by exposing to a chlorine plasma and then chemically etched using HCl or other solutions [34]. Temperatures ranging from 25–250° C. were utilized by Kuo and Lee with the higher temperatures being more effective. Surface roughness along the sidewalks appears to be an important issue in their process.
One of the problems in understanding the etching process in the Cu—Cl system is that it is not clear which Cu gaseous species is responsible for the primary etching mechanism. It has been reported that the trimer Cu3Cl3 is the dominant gas species in this system [5,23], however the precise conditions (i.e., chlorine partial pressure and temperature) for which this species dominates appears to have not been analyzed systematically. The difficulty arises because there are a large number of gaseous (Cu(g), CuCl(g), CuCl2(g), Cu3Cl3(g)), etc.) and condensed (solid) phase species (Cu(c)*, CuCl(c), CuCl2(c)) in this system. Experiments involving mass spectrometric measurements of the partial pressures of gases along with other techniques to determine the condensed phase species are certainly helpful, however these can be quite tedious. Theoretical approaches based on the thermodynamics ofthis system may be preferable provided the thermodynamic data is available. Fortunately, for the Cu—Cl system, the thermodynamic data has been assessed and is available through a number of sources, for e.g., JANAF tables [35] or databases accompanying thermodynamic software such as HSC Chemistry [36] and FACT [37]. In spite of this, it is still not a trivial task sorting through the list of reactions and determining the dominant condensed and vapor phase species at various temperatures and chlorine partial pressures. A graphical representation is probably the best approach. Conventional phase diagrams, Richardson-Ellingham diagrams and Pourbaix diagrams [38] are a few examples of such graphical representations that are useful in various areas of research. In the present situation, an appropriate graphical method for representation of solid-gas reactions, so as to understand etching mechanisms, is a “volatility diagram.” Volatility diagrams are typically used in the high-temperature industry to examine volatility behavior of materials such as refractories and ceramics when exposed to high temperatures and reactive environments [39–44]. The earliest works in this field appear to be those of Wagner (39) who utilized such a diagram for analyzing the active-passive oxidation of silicon. Gulbransen and Jansson (40–42) used these diagrams (also known as thermochemical diagrams) for the analysis of volatility behavior of refractory metals (40), ceramics (41) and liquid metals (42). Typically, in a volatility diagram, for example the Si—O system, the partial pressures of the important volatile species that contain the solid element (e.g., Si(g), SiO(g)) are plotted as a function of the partial pressure of oxygen at various temperatures. A comprehensive review of the construction and application of volatility diagrams for ceramic materials is given in the paper by Lou, Mitchell and Heuer (43, 44). The present work extends the use of volatility diagrams for understanding dry etching mechanisms in the Cu—Cl system.